1. Field of the Invention
The present invention relates to a device for reducing cross-talk in an optical disk device, in which an astigmatism method is used for sensing focusing errors.
2. Description of the Related Art
An optical disk device is constructed such that a luminuos flux outputted from a laser source is focused and formed as a spot on an optical disk that is used as a recording medium, whereby a reproduction and recording of a signal is carried out. In this case, to carry out an accurate recording and reproduction of the signal, it is necessary to converge a laser beam on an optical disk as a spot having a diameter which approximates a diffracting limit of the beam, so that the laser beam exactly traces data on a recording track of the optical disk. Accordingly, the optical disk device is provided with an error sensing system for sensing focusing errors and tracking errors, and a drive mechanism for driving the laser beam spot along the optical axis of the optical system and in the radial direction of the optical disk.
Among the known methods of sensing focusing errors, the astigmatism method is most generally used. In the astigmatism method, a beam reflected from an optical disk is converged on a photo sensor by using an element, such as a cylindrical lens, having an astigmatic difference, so that a defocusing of the spot converged on the optical disk is determined as a change of a sectional shape of the luminous flux on the photo sensor.
The photo sensor that is is provided with a square-shaped sensor divided into four photo receiving areas by lines which are perpendicular to each other, and pass through and cross each other at the center of the square-shaped sensor. The focusing error signal is obtained by calculating a difference of each of the sums of signals outputted from the photo receiving areas located diagonally on the surface of the square-shaped sensor, i.e., at opposite left and right upper and lower corners thereof, respectively.
In the optical disk, device using an optical disk such as a photo magnetic disk, a beam spot is traversed across one track, to thereby access another track at which recording or reproduction is to be carried out. When a sensing of focusing errors is carried out in accordance with the astigmatism method, however, if a luminous flux converged on the optical disk is affected by a wave aberration, due to a characteristic of the optical system, noise is added to the focusing error signal when the spot is moved across a track.
The noise is caused by an uneven intensity of the light of the beam spot on the photo sensor, due to a diffraction of the light occurring when a spot having a wave aberration traverses a wall of a groove forming a boundary between adjacent tracks. In this specification, this noise is defined as focus-track cross-talk (or F-T- cross-talk) hereinafter.
When focus-track cross-talk occurs, a focusing error signal is generated which appears to denote that the objective lens has deviated from an in-focus position. Accordingly, the objective lens is moved in the optical axis direction. Therefore, the objective lens is in an unstable state every time the spot traverses a track, and this leads to errors in the movement of the spot.